For many years there has been recognition that vehicles could be made more fuel-efficient if the energy normally lost in decelerating or braking the vehicle could be somehow collected, stored and reused to accelerate the vehicle. A relatively large number of prior patents and published patent applications exist which are directed to various aspects of this general approach. Some have proposed to collect and store the energy in hydraulic accumulators and then reuse the energy through fixed or variable displacement hydraulic transmissions. The recovered energy was used to assist or provide vehicle motion.
Some vehicles, such as refuse trucks, had auxiliary hydraulic circuits that were powered by the vehicle's engine through a power take-off unit. Consequently, it was necessary for the engine to be running to operate the auxiliary hydraulic systems on the truck. Typical auxiliary hydraulic circuits included those use to actuate cylinders, power hydraulic motors (other than those associated with vehicle propulsion), etc.